In an effort to maintain a high degree of efficiency, manufacturers of turbine engines have strived to maintain the closest possible clearance between a rotor blade tip and the surrounding stationary shroud structure, because any gas which passes therebetween represents a loss of energy to the system. If a system were to operate only under steady-state maximum power conditions, it would be a simple matter to establish the desired close clearance relationship between the rotor blades and the surrounding stationary shroud. However, in reality, all turbine engines must initially be brought from a standstill condition up to steady-stat speed and then eventually decelerate to the standstill condition.
This transitional operation is not completed with the ideal low clearance condition just described. The problems in maintaining the desired clearance between the rotor and shrouds under these transitional conditions are caused by first, the mechanical expansion and shrinkage of the rotating rotor disk and blades as brought about by changes in speed, and secondly, by the relative thermal growth between the rotating rotor and surrounding stationary shroud support structure caused by differences in thermal expansion between the two structures. One commonly used method of decreasing the tip clearance between the rotor blades and the surrounding shroud has been to direct and modulate variable temperature air or variable cooling airflow rates along the entire outer circumference of the stationary shroud support structure. In this method, the air is directed on the turbine section during appropriate stages of engine operation to change the radial growth or shrinkage rate of the entire turbine shroud support in an effort to match the growth or shrinkage of the rotating turbine parts.
However, additional problems occur during an aircraft maneuver, such as during takeoff and landing. During these maneuvers, engine loadings develop that become eccentric to the engine centerline. One common method of minimizing the clearance effects of eccentric loadings is to eccentrically grind the stationary surrounding shroud, as is shown in FIG. 3. However, this method results in additional airflow leakage around the rotor blades during steady-state, low maneuver load conditions as a result of the added clearance between the rotor blades and a portion of the surrounding shroud.